Plastic bags are presently manufactured by continuous movement of a tubular or folded plastic web through a bag making machine or apparatus. The web is generally stored in a large supply roll of a flat film and is drawn and folded under controlled tension, usually through driven draw rolls of the bag machine apparatus. A cut and seal unit is mounted downstream of the draw rolls and severs the folded web transversely to form a series of bags of a selected or standard length. A transfer unit, often in the form of a wicketer, is provided for transfer of the bags to a wicket stacking unit or device. The wicketer includes a plurality of circumferentially spaced vacuum arms secured to a rotating device or support. The bag from the cut and seal unit is held to an arm which rotates from an input side to a discharge side and deposits the bags on the stacking device. A widely used stacking device includes an endless wicket conveyor with a motor and drive connected at the outlet end. Pin stacker units are secured to an endless stacker support member in spaced relation for selective positioning between the input end of the conveyor and the output or discharge end of the conveyor. The conveyor input end aligns a pin stacker unit with the movement of the wicket arms at the discharge side of the wicketer. As each arm moves past the pin stacker, the bag is deposited onto the pins of the pin stacker. The bag is formed with one or more appropriately spaced openings which are aligned with the pin or pins on the pin stacker.
Generally, each bag stack will have a selected number of bags to produce a "filled" pin stacker. The conveyor is operated to remove the filled stacker and move a new pin stacker automatically aligned with the discharge side of the wicketer. Movement of the filled stacker requires a greater period of time than that required for the movement of adjacent vacuum arms into an aligned position. Historically, the bag forming portion of the line is interrupted to allow movement of one or more interrupt cycles and empty vacuum arms move through the input end of the conveyor and allow the movement of the succeeding or new stacker element into position to receive the bags from the arms following the interrupt arms. In this manner, an essentially continuous operation of the bag making machine or apparatus provides for sequential forming and accumulation of stacks of corresponding bags. Each stack, of course, is discharged or removed at the discharge and output end of the conveyor, either through an automatic or manual removal system.
The bag making machinery or apparatus is operated at a maximum operating speed permitted by the several components to produce a most cost effective forming of the bags. Obviously, the required time for repositioning of a filled pin stacker and replacement with a new pin stacker may be a limiting factor in the total overall production of bags per unit of time.
Chain driven conveyors have generally been used in the wicket conveyor. Stacking platforms are secured to the chain in longitudinally spaced relation, with the pins adjustably secured to the platform to accommodate different forms of bags. A preferred construction is shown in the pending patent application assigned to a common assignee and entitled "Bag Forming Machine Having Adjustable Support Structure For Paired Work Elements", inventor Michael J. Smith et al with Ser. No. 08/600,341 and filed Feb. 13, 1996. An independent drive unit is secured to the discharge and output end of the conveyor chain drive and operated in time spaced relation through a timing control associated with the interruption of the bag forming part of the machine. In the prior art wicketing conveyor, the conveyor chain drive is mounted in a slide support for positioning the pin stackers in bag receiving alignment. The drive unit includes a geared adjustment motor and positioning coupling for moving the complete chain unit for such alignment positioning and is a relatively large unit at the outer end of the conveyor. In this conveyor drive system, the conveyor chain unit is pulled forwardly and must be concerned with the slack of the unit and over shooting and/or oscillating thereof.
Historically, the independent drive motor is secured to the discharge end of the conveyor and the initiation of the conveyor motor operation is controlled from a control system which also actuates the bag forming machine. Thus, the draw rolls for moving of the web is operated in an intermittent and interrupted manner and is controlled to stop movement of the web during selected movement of the wicketer to allow transfer of one or more empty wicketer arms through and to the conveyor. For example, for many years a logic controller was connected through a clutch and brake control for actuating of the draw roll drive. The timing control was established through a main drive shaft driven from an AC drive motor. A cam unit coupled the main drive shaft to the cut and seal unit and a programmable limit switch provided a reference source for controlling of the draw rolls and the wicket conveyor. Thus, the output of the drive shaft provided a cycle control. Each 360.degree. rotation of the drive shaft created one cycle of the bag forming machine. The web was drawn by the draw rolls into appropriate alignment with the seal and cut unit. Movement of the draw rolls was then interrupted momentarily to allow the seal and cut unit to sever the web and produce a bag which was discharged to a wicketer for transfer to a wicket conveyor. A stack count was generated by this cyclical operation. A logic controller included a plurality of registers, one of which provided an interrupt count and a second provided a delay count. When the stack count indicated that the number of bags equal to a stack had been formed, the bag forming machine was signaled for interrupt operation for the necessary time for the transfer unit to transfer formed bags to the stacker and allow insertion of a new stacker. An interrupt count was set to create empty wicket arms of a sufficient number and period to allow the operation of the wicket conveyor through a separate, independent drive. A signal was sent to the conveyor motor drive after an appropriate time delay as set by the second register to allow the transfer of all formed bags to the stacker and then to initiate the cycle of wicket conveyor during movement of the bag-free arms past the input end of the conveyor. A photocell unit, or other as sensor, may be coupled to the input end of the wicket conveyor as in the prior art and generate pulse signals which would detect a jammed condition and also provide the signals to the conveyor register. The independent conveyor motor drive once started, included a self-controlled cycle with an index complete limit switch controlling the distance of movement of the wicket conveyor to move the new pin stacker into alignment as well as the time within the indexing cycle at which the motor starts to synchronize the conveyor for arrival one or more empty arms, at which time the cycle would repeat.
With the development of the servo motors, and particularly AC servo motors, various drives for the draw rolls, the seal unit and the wicketer and the conveyor had been developed and applied.
For a number of years, the assignee of this invention has manufactured and sold machines using a logic control system with AC servo motors for operation of the various components of the bag forming machine and wicketer. In each instance, an independent motor drive for operating of the wicket conveyor was provided to allow and maintain operating control. A jam detector which develops a pulse per bag movement, was also used, not only for detecting jams, but to synchronize the conveyor indexing with the operation of the draw rolls. Thus, the system allowed the usual drive of a counter register of the control system from the a pulse generator coupled to a main drive shaft or from the jam detector to initiate the new cycle of the draw rolls. In this system, a register is provided to delay the operation of the independent conveyor motor, at which time a signal was sent to a starting relay which initiated the starting of the independent motor drive, which then completed its cycle. The assignee has used a multi-axis servo controller for operating of the draw roll, the seal and cut unit and the vacuum wicketer. Registers were then driven from the main pulse source or from the jam detector sensor. The one register incremented to count the interrupt cycles. The second register incremented a preset number of cycles to initiate the operation of the independent motor drive for the wicket conveyor. In a typical operation of a six armed vacuum unit, three cycles were counted prior to beginning indexing of the wicket conveyor to allow transfer of the three last formed bags created after interruption of the bag forming machine or apparatus.
U.S. Pat. No. 5,338,281, which issued Aug. 16, 1994, discloses a single multi-axis servo-controller for operating of all of the components of a bag line including the wicket conveyor. The single controller controls the draw rolls and the conveyor including initiation and termination thereof as well as each component of the system.
There is a continuing need for a system to provide accurate and rapid positioning of the wicket conveyor for alignment of the pin stackers for receiving the bags. A more compact bag line adapted to multiple lines system is desirable.